<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title></title>
    <style>
        *{
            margin: 0;
            padding: 0;
            list-style: none;
            border:none;
        }

        #box{
            width: 800px;
            height: 200px;
            border: 1px solid #ddd;

            position: relative;
            margin: 100px auto;

            overflow: hidden;
        }

        #box ul{
            width: 2600px;
            position: absolute;
            left: 0;
            top: 20px;
        }

        #box ul li{
            float: left;
        }

        #box_bottom{
            position: absolute;
            left: 0;
            bottom: 0;
            background-color: #e8e8e8;

            width: 100%;
            height: 25px;
        }

        .mask{
            position: absolute;
            left: 0;
            top:0;
            height: 25px;
            background-color: orangered;
            border-radius: 12px;
            cursor: pointer;
        }
    </style>
</head>
<body>
    <div id="box">
        <ul id="box_top">
            <li><img src="images/img1.jpg" alt=""></li>
            <li><img src="images/img2.jpg" alt=""></li>
            <li><img src="images/img3.jpg" alt=""></li>
            <li><img src="images/img4.jpg" alt=""></li>
            <li><img src="images/img5.jpg" alt=""></li>
            <li><img src="images/img6.jpg" alt=""></li>
            <li><img src="images/img7.jpg" alt=""></li>
            <li><img src="images/img8.jpg" alt=""></li>
            <li><img src="images/img1.jpg" alt=""></li>
            <li><img src="images/img2.jpg" alt=""></li>
            <li><img src="images/img1.jpg" alt=""></li>
            <li><img src="images/img2.jpg" alt=""></li>
            <li><img src="images/img3.jpg" alt=""></li>
            <li><img src="images/img4.jpg" alt=""></li>
            <li><img src="images/img5.jpg" alt=""></li>
            <li><img src="images/img6.jpg" alt=""></li>
            <li><img src="images/img7.jpg" alt=""></li>
            <li><img src="images/img8.jpg" alt=""></li>
            <li><img src="images/img1.jpg" alt=""></li>
            <li><img src="images/img2.jpg" alt=""></li>
        </ul>
        <div id="box_bottom">
            <span class="mask"></span>
        </div>
    </div>

<script>
    window.onload = function () {
        // 1. 获取需要的标签
        var box = document.getElementById("box");
        var box_top = document.getElementById("box_top");
        var box_bottom = document.getElementById("box_bottom");
        var mask = box_bottom.children[0];

        // 2. 设置滚动条的长度
        // 滚动条长度 = ( 盒子的宽度 / 内容的宽度) * 盒子的宽度
        var mask_len = (box.offsetWidth / box_top.offsetWidth) * box.offsetWidth;
        mask.style.width = mask_len + 'px';

        // 3. 鼠标操作
        mask.onmousedown = function (event) {
            var e = event || window.event;

            // 3.1 求出初始值
            var beginX = e.clientX - mask.offsetLeft;

            // 3.2 移动
            document.onmousemove = function (event) {
                var e = event || window.event;

                // 3.3 求出移动的距离
                var endX = event.clientX - beginX;

                // 边界值
                if(endX < 0){
                    endX = 0;
                }else if(endX >= box.offsetWidth - mask.offsetWidth){
                    endX = box.offsetWidth - mask.offsetWidth;
                }


                // 3.4 动起来
                mask.style.left = endX + 'px';

                // 内容走的距离 = （内容的长度 - 盒子的长度） / (盒子长度 - 滚动条的长度) * 滚动条走的距离
                var content_len = (box_top.offsetWidth - box.offsetWidth) / (box.offsetWidth - mask.offsetWidth) * endX;
                box_top.style.left = -content_len + 'px';

                return false;
            };

            document.onmouseup = function () {
                document.onmousemove = null;
            }
        }
    }
</script>
</body>
</html>